1. Technical Field
This disclosure relates generally to an image forming apparatus, and more specifically, an image forming apparatus having a recording head in which a plurality of head units are arranged in a direction perpendicular to a conveyance direction of a recording medium.
2. Description of the Related Art
An image forming apparatus used as a printer, facsimile machine, copier, or multi-functional device thereof may have a liquid discharge device including a recording head configured as, for example, a liquid discharge head for discharging liquid droplets of a recording liquid, for example, ink. Such image forming apparatuses perform image formation by attaching liquid droplets onto a recording medium (hereinafter “sheet” or “sheets”).
For such image forming apparatuses, various line-type recording heads have been developed to enhance printing speed. One conventional line-type recording head includes a plurality of head units, which may be called “head chips.” Such head units include a plurality of nozzles arrayed in one or more rows in a given direction (hereinafter “nozzle array direction”). In such a line-type recording head, for example, the plurality of head units may be staggered in a direction perpendicular to a conveyance direction of a sheet so that the nozzle array direction in each of the head units is perpendicular to the sheet conveyance direction.
Alternatively, the plurality of head units may be inclined with respect to the sheet conveyance direction so that a nozzle array direction in each head unit is inclined at a certain angle to the sheet conveyance direction.
For example, in one conventional line-type recording head, a plurality of head units are arranged so that adjacent head units overlap by, for example, several nozzles in a direction perpendicular direction to a sheet conveyance direction. Alternatively, a plurality of head units may be arranged so that end columns of nozzles of adjacent heads overlap in the direction perpendicular direction to the sheet conveyance direction.
Such line-type recording heads may include a plurality of head units having a relatively short length. Accordingly, such line-type recording heads may be manufactured at a relatively high yield and thus at a relatively low cost compared to a single long head in which the width of a nozzle array is greater than the width of a sheet.
However, for such a configuration, in a border area in which liquid droplets discharged from adjacent head units are joined together in an image, deviation in landing positions of the liquid droplets may show up as white or black streaks, thereby degrading image quality.
Hence, in order to prevent such white or black streaks due, for example, to thermal expansion of the recording head, one conventional line-type recording head includes a plurality of head units, in each of which a plurality of nozzle arrays are staggered so as to overlap for several nozzles in a direction perpendicular to a sheet conveyance direction, and selects whether to use nozzles in such overlapping areas depending on the degree of thermal expansion of the recording head.
As described above, an image forming apparatus employing a liquid discharge system typically discharges liquid droplets from a recording head to form an image on a sheet. When such liquid droplets adhere to the sheet the sheet may swell, thereby resulting in cockling, that is, a phenomenon in which the sheet is deformed into a wavy shape, which further may cause landing position deviation as described above.
As a result, certain conventional image forming apparatuses include a guide member, which may be called a platen, having ribs disposed opposite a recording head to support the sheet from below. Such guide member may be used to keep the sheet substantially flat during conveyance of the sheet.
Alternatively, one conventional inkjet recording apparatus includes a platen that deforms a sheet into a wavy shape along a direction perpendicular to a sheet conveyance direction. The platen also deforms the sheet using irregularities formed by a plurality of ribs arranged parallel to the sheet conveyance direction. Such conventional inkjet recording apparatuses also include projections that guide the sheet downward to recesses formed between the ribs of the platen, and prevent the sheet from floating toward a recording head side due to swelling.
Alternatively, in one conventional technique, a platen has a first rib extending in a sub-scanning direction and a second rib extending in the sub-scanning direction on a face opposite a recording head. The first rib is formed higher than the second rib in a direction toward the recording head, and is provided so as not to project out to a recording head side across a hypothetical line that passes through a nip between conveyance rollers.
In another conventional technique, the distance between a sheet and a liquid discharge face of a recording head is kept substantially constant by tops of ribs supporting the sheet from below in an area outside of a landing area of liquid droplets. In such a landing area of liquid droplets, the tops of ribs have a height such as not to contact the sheet from below. Alternatively, the platen is formed so as not to have such ribs in the landing area of liquid droplets.
However, in conventional image forming apparatuses with a line-type recording head in which a plurality of head units are arranged in one or more rows, liquid droplets discharged from respective end nozzles of adjacent head units may not land at a constant pitch in a border area between such adjacent head units. Consequently, a deviation in landing position and/or color may occur in such border area.
Further, if cockling occurs on a printed sheet, the distance between the sheet and one head unit provided on an upstream side in a sheet conveyance direction may differ from the distance between the sheet and another head unit on a downstream side in the sheet conveyance direction. Consequently, even when liquid droplets are discharged from adjacent head units with a normal timing, i.e., a timing suitable for a flat state of sheet, landing position deviation may occur in a border area between such liquid droplets, thereby degrading image quality.
For example, as illustrated in FIG. 1, assume that a head base 501 is provided with a head unit 502A and a head unit 502B on upstream and downstream sides, respectively, in a sheet conveyance direction Y. In such case, a liquid droplet “A” indicated by a white circle and a liquid droplet “B” indicated by a black circle are discharged from respective nozzles of the head unit 502A and the head unit 502B.
At this time, when a sheet 503 is conveyed at a normal flat state indicated by a dot-and-dash line 504 without being subjected to cockling in the sheet conveyance direction Y, liquid droplets are adjusted so that a landing position DA of the liquid droplet A and a landing position DB of the liquid droplet B may be precisely aligned adjacent to each other in a direction substantially perpendicular to the sheet conveyance direction Y as indicated by dashed circles in FIG. 2.
However, if the sheet 503 is deformed into a wave shape due to cockling as illustrated in FIG. 1, the liquid droplet A discharged from one nozzle of the head unit 502A may land on a lower portion of the sheet 503. At this time, the distance between the head unit 502A and the lower portion of the sheet 503 is longer than a normal distance therebetween. As a result, even when the liquid droplet A is discharged with a normal timing, the liquid droplet A lands on the sheet 503 with a later timing than when the sheet 503 is flat. Consequently, the landing position DA1 of the liquid droplet A deviates from the normal landing position DA in a direction opposite the sheet conveyance direction Y.
Meanwhile, the liquid droplet B discharged from one nozzle of the head unit 502B may land on a higher portion of the sheet 503 generated by such wavy deformation. At this time, the distance between the head unit 502B and the convex portion of the sheet 503 is shorter than a normal distance therebetween. As a result, when the liquid droplet B is discharged with a normal timing, the liquid droplet B lands on the sheet 503 with an earlier timing than when the sheet 503 is flat. Consequently, the landing position DB1 of the liquid droplet B deviates from the landing position DB1 of the liquid droplet B in the same direction as the sheet conveyance direction Y.
Further, even a slight amount of landing position deviation in a sheet conveyance direction occurring between liquid droplets discharged from adjacent different head units may have a noticeable effect on the resultant image, thereby degrading the image quality.
When swelling occurs in a sheet due to cockling, the sheet is deformed so as to extend in a direction perpendicular to a sheet conveyance direction, which may result in a landing position deviation as described above. In such case, the amount of deviation may become significantly large in areas where the plurality of head units overlap. Consequently, white or black streaks may be easily generated in such areas, thereby degrading image quality.